what I mean is the most difficult to feed i.e. the filament (any diameter) that jams the most.

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So much depends on the machine, its extruder and the way it was assembled, the nozzle temperature, feed rate, retraction, filament composition and many other variables. Different people get different results for the same filament on different machines but, in general, flexible filament is the most difficult and particularly so for 1.75mm in a Bowden system. All other filaments are OK provided you are sensible about the limitations of your machine and the task you are undertaking.

obviously the machine and feeder are important to results as is speed, temp etc etc. and flex is more difficult than stiff and 1.75 is more difficult than 2.8

what I am trying to find out... from as many people as possible.... is what SPECIFIC filament has caused the WORST FEED problem.... in ANY machine (NOT having a pop at anyone, esp mike01hu)

I am looking at feed mechanisms and designing one for difficult filaments.... therefore machine, feeders, direct or bowden etc is superfluous.... which filament has caused YOU the worst problems... NOT the print quality, simply the problem FEEDING it.... eg devil flex extra bendy (I made up that name, I hope)

I believe that there are all kinds of good extruders and hot ends out there already. The next thing to work on is PART COOLING that is the most significant thing to work on to improve part quality. And everybody uses patched up hacks and home made crappy stuff.
I am thinking of something like the E3D silicone boot for the hotend WITH 2 30mm fan shrouds built in to it...

the titan and V6 have pretty much eliminated all issues with filament feeding. we print 1.75mm flexibles out of our volcano with a titan and we get beautiful prints out of it, even doing very large 800g prints that take 20+ hours to print. I dont see any reason to try to develop something like this when an amazing solution already exists